I save RAW (CR2) files to a folder on my hard drive. I then import/copy them to the catalog (No DNG conversion). Most of the files are 18-20meg. However, when I mouse over the image in Library, it shows most of them as 15meg, but still CR2. What's happening here? I'm also not creating previews (yet), in most cases. Also I'm using LR 5.5 on OS10.9.

Yes, I need to learn how to store/organize photos more efficiently. In the meantime, I want to make sure I've got full access to the data for editing. Afterall, that's why I'm shooting in RAW.

The megapixels is determined by the width and height of the sensor data and would be relatively constant for a particular raw setting, whereas the megabytes would vary based on the compressibility of both the raw data and embedded JPG preview which are more compressible if there are smooth areas and less noise due to ISO settings, so the size of the files, themselves can vary widely.

Thanks to both you guys for your replies. I knew I had exact copies on my hard drive; just couldn't figure out how/why the LR images were somehow "smaller".

So, ssprengel, yes, I was confusing MP with megabytes. Jeesh. Whenever I would mouse over a thumbnail, I'd get a small yellow "sticky note" box with details such as the file name, total MP, and the pixel dimensions. Though I know my camera's got a 15MP sensor, for the life of me I couldn't figure out the discrepancy with the file size. Now I got it. Such a newbie mistake, but it was really throwing me off.